2,745 research outputs found
Enhanced electroweak penguin amplitude in B-->VV decays
We discuss a novel electromagnetic penguin contribution to the transverse
helicity amplitudes in B decays to two vector mesons, which is enhanced by two
powers of mB/Lambda relative to the standard penguin amplitudes. This leads to
unique polarization signatures in penguin-dominated decay modes such as B-->rho
K* similar to polarization effects in the radiative decay B-->K*gamma, and
offers new opportunities to probe the magnitude and chirality of
flavour-changing neutral current couplings to photons.Comment: 4 pages, 1 figur
Ab initio structure modeling of complex thin-film oxides: thermodynamical stability of TiC/thin-film alumina
We present an efficient and general method to identify promising candidate
configurations for thin-film oxides and to determine structural characteristics
of (metastable) thin-film structures using ab initio calculations. At the heart
of this method is the complexity of the oxide bulk structure, from which a
large number of thin films with structural building blocks, that is motifs,
from metastable bulk oxide systems can be extracted. These span a coarse but
well-defined network of initial configurations for which density functional
theory (DFT) calculations predict and implement dramatic atomic relaxations in
the corresponding, resulting thin-film candidates. The network of thin-film
candidates (for various film thicknesses and stoichiometries) can be ordered
according to their variation in ab initio total energy or in ab initio
equilibrium Gibbs free energy. Analysis of the relaxed atomic structures for
the most favored structures gives insight into the nature of stable and
metastable thin-film oxides. We investigate ultrathin alumina nucleated on TiC
as a model system to illustrate this method.Comment: Submitted to PRB; 16 pages, 11 figure
Balanced sensitivity and specificity on unbalanced data using support vector machine re-thresholding
Support vector machine (SVM) classifiers use multivariate patterns to separate two groups by a hyperplane with maximal margin. This strategy tends to obtain good generalisation accuracy on even very high dimensional applications. However, SVMs are not well suited to unbalanced data with very different numbers of cases in each group. In this work we implement a properly cross-validated method for altering the SVM threshold (also known as the bias or cut-point) to re-balance the sensitivity and specificity
Understanding adhesion at as-deposited interfaces from ab initio thermodynamics of deposition growth: thin-film alumina on titanium carbide
We investigate the chemical composition and adhesion of chemical vapour
deposited thin-film alumina on TiC using and extending a recently proposed
nonequilibrium method of ab initio thermodynamics of deposition growth (AIT-DG)
[Rohrer J and Hyldgaard P 2010 Phys. Rev. B 82 045415]. A previous study of
this system [Rohrer J, Ruberto C and Hyldgaard P 2010 J. Phys.: Condens. Matter
22 015004] found that use of equilibrium thermodynamics leads to predictions of
a non-binding TiC/alumina interface, despite the industrial use as a
wear-resistant coating. This discrepancy between equilibrium theory and
experiment is resolved by the AIT-DG method which predicts interfaces with
strong adhesion. The AIT-DG method combines density functional theory
calculations, rate-equation modelling of the pressure evolution of the
deposition environment and thermochemical data. The AIT-DG method was
previously used to predict prevalent terminations of growing or as-deposited
surfaces of binary materials. Here we extent the method to predict surface and
interface compositions of growing or as-deposited thin films on a substrate and
find that inclusion of the nonequilibrium deposition environment has important
implications for the nature of buried interfaces.Comment: 8 pages, 6 figures, submitted to J. Phys.: Condens. Matte
Spin-Correlation Coefficients and Phase-Shift Analysis for p+He Elastic Scattering
Angular Distributions for the target spin-dependent observables A,
A, and A have been measured using polarized proton beams at
several energies between 2 and 6 MeV and a spin-exchange optical pumping
polarized He target. These measurements have been included in a global
phase-shift analysis following that of George and Knutson, who reported two
best-fit phase-shift solutions to the previous global p+He elastic
scattering database below 12 MeV. These new measurements, along with
measurements of cross-section and beam-analyzing power made over a similar
energy range by Fisher \textit{et al.}, allowed a single, unique solution to be
obtained. The new measurements and phase-shifts are compared with theoretical
calculations using realistic nucleon-nucleon potential models.Comment: Submitted to Phys. Rev.
A 10 GHz Y-Ba-Cu-O/GaAs hybrid oscillator proximity coupled to a circular microstrip patch antenna
A 10 GHz hybrid Y-Ba-Cu-O / GaAs microwave oscillator proximity coupled to a circular microstrip antenna was designed, fabricated and characterized. The oscillator was a reflection mode type using a GaAs MESFET as the active element. The feedline, transmission lines, RF chokes, and bias lines were all fabricated from YBa2Cu3O(7-x) superconducting thin films on a 1 cm x 1 cm lanthanum aluminate substrate. The output feedline of the oscillator was wire bonded to a superconducting feedline on a second 1 cm x 1 cm lanthanum aluminate substrate, which was in turn proximity coupled to a circular microstrip patch antenna. Antenna patterns from this active patch antenna and the performance of the oscillator measured at 77 K are reported. The oscillator had a maximum output power of 11.5 dBm at 77 K, which corresponded to an efficiency of 10 percent. In addition, the efficiency of the microstrip patch antenna together with its high temperature superconducting feedline was measured from 85 K to 30 K and was found to be 71 percent at 77 4 increasing to a maximum of 87.4 percent at 30 K
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